1. Field of the Invention
This invention relates to size reduction machines and, in particular, to a series of interchangeable screens for use in size reduction machines, without any adjustment in gap size being required and to an apparatus for adjusting the size of a gap between an impeller and a screen.
2. Description of the Prior Art
Previous size reduction machines utilize a frustoconical shaped screen located in a channel between an input and output. Various screens and impellers are used in the machine depending on the size and type of particles that are being processed. Different screens have apertures of different sizes and shapes. Sometimes, size reduction is accomplished in stages by successively using screens with smaller apertures. The thickness of the material used to construct a particular screen generally varies with the size of the apertures required. Previous screens are made in a conical die in such a manner that the interior height of each screen made on the same die varies with the thickness of material being used. This variation in interior height can result in confusion in operating the size reduction machines as the gap between the impeller and the interior surface of the screen varies when one screen is replaced with a different screen having a different wall thickness. The different wall thickness is compensated for by inserting or removing spacers on the impeller shaft to move the impeller relative to the interior of the screen. However, as a side wall of the screen is tapered relative to the impeller, the actual adjustment of the gap is less than the thickness of the spacer and depends upon the angle of the screen relative to the horizontal. Where the tapered wall of the screen has an angle of 60.degree. relative to the horizontal, the gap is adjusted by one-half the thickness of the spacer. In other words, if it is desired to adjust the gap by 10 mm, a spacer of 20 mm is inserted or removed from the impeller shaft in order to maintain a constant gap. This adjustment of the gap becomes more complicated where it was desired to use a new screen having a different wall thickness and at the same time increase the gap size. Then, a calculation must be done to determine the size of spacer necessary to compensate for the change in wall thickness and also for the desired change in gap size. With previous size reduction machines, it is usually necessary to remove or insert spacers on the impeller shaft whenever a screen having a different wall thickness from the previous screen is used in the machine. If the angle of the tapered wall is something other than 60.degree. the relationship between the gap size and material thickness of the screen can be even more complex.
Further, with previous size reduction machines, difficulties can be encountered in changing the gap size as the change is accomplished by inserting or removing spacers, of an appropriate size, on the impeller shaft. This usually necessitates removing the screen from the machine as well as removing the impeller. A spacer or spacers of appropriate size are then removed from or inserted onto the shaft and the impeller and screen are replaced. Depending on the type of particles that are being processed, the screen and impeller can be very messy, thereby causing additional problems when it is desired to change the gap size. In some uses of size reduction machines, it is desirable to change the gap size without changing the screen. The same procedure as that outlined above for inserting or removing spacers is then followed.